Radar level gauge systems are in wide use for measuring process variables of a product contained in a tank, such as filling level, temperature, pressure etc. Radar level gauging is generally performed either by means of non-contact measurement, whereby electromagnetic signals are radiated towards the product contained in the tank, or by means of contact measurement, often referred to as guided wave radar (GWR), whereby electromagnetic signals are guided towards and into the product by a probe acting as a waveguide. The probe is generally arranged vertically from top to bottom of the tank. The electromagnetic signals are subsequently reflected at the surface of the product, and the reflected signals are received by a receiver or transceiver comprised in the radar level gauge system. Based on the transmitted and reflected signals, the distance to the surface of the product can be determined.
In many applications, it is also desirable to be able to determine the position of one or more interfaces between different materials in a tank in order to accurately determine the distribution of material in a tank.
GWR systems have found a widespread use for the measurement of interfaces between different materials contained in a tank, such as for example the interface between oil and water.
An abrupt interface between two materials having different dielectric constants causes a portion of an electromagnetic signal propagating along a probe to be reflected, thereby the position of the interface can be determined based on the time of flight of the reflected signal and the propagation properties of the probe.
However, for some material combinations or environmental conditions, the interface may comprise an emulsion of or a mixture between materials, thereby resulting in an extended interface instead of an abrupt. An extended interface may also be seen as a diffuse interface or a transition zone.
An extended interface may lead to an unpredictable reduction of amplitude of the reflected signal as it propagates through the interface. Depending on the thickness of the upper layer, the signal resulting from a reflection at an interface may be further reduced as a result of dielectric losses induced in the transmission line by the upper layer. Consequently, a diffuse interface may result in a reflected signal which is reduced in amplitude or otherwise distorted to the extent that it is no longer possible to detect the interface.
Furthermore, even if a detectable reflected signal is received, there may be an uncertainty in determining the position of a diffuse interface as it is not known where in the extended interface the electromagnetic signal is reflected.
US2010/0313654 discloses a method for determining the position of an extended interface between two products in a tank by using a combination of a travel time measurement for an electromagnetic signal reflected at the interface and a capacitance measured between a capacitive probe and reference electrode.
However, the method according to US2010/0313654 requires that the interface is sufficiently well defined such that a detectable reflection of the electromagnetic signal occurs. Furthermore, as a second measurement involving capacitance measurement is required, system complexity is increased.